This invention relates to permanent magnet structures for generating very strong yet highly uniform magnetic fields, primarily but not exclusively for use in medical applications of nuclear magnetic resonance.
Medical applications of nuclear magnetic resonance (NMR) are primarily based on the use of either superconductive magnets or permanent magnets to generate the highly uniform field required for imaging. The main advantage of superconductive magnets is their ability to reach high magnetic fields. The superconductive coils that carry the electric current are normally arranged in cylindrical structures open at both ends, and the patient is inserted axially to access the imaging region located at the center of the magnet. This arrangement, dictated by the geometry of the coils, generates a number of problems in clinical as well as surgical applications. For instance, real time imaging during a surgical procedure is hampered by the interference of the magnet structure with the surgical instrumentation and by the restricted access to the patient and to the surgical area in particular.
To alleviate these problems, in recent designs based on superconductive magnets as described in "A System for MRI-guided interventional procedures" P. B. Reemer, J. F. Schenck, F. A. Jolesz et al. Proceedings of II.sup.nd meeting; Society at Magnetic Resonance, Vol. 1., p. 420, the surgical area is positioned outside the superconductive coils. These approaches require the coil dimensions to be large compared to the body dimensions in order to achieve the desired degree of field uniformity within the surgical area. Furthermore, a field much larger than the one required within the imaging region is generated outside the region of interest and outside the magnet itself.
The use of permanent magnets that require no external power supply and no maintenance is gaining momentum in medical imaging, in spite of their limitations in the generation of high fields. Traditional permanent magnets can be designed with yoke geometries that leave a wide open area around the imaging region contained within the gap between the pole pieces. However, to keep the magnet size and its weight within practical limits, the pole pieces must be as close as possible to the body. The large transverse dimensions of the pole pieces dictated by the required field uniformity within the region of interest again limits the access to the patient and interferes with the surgical instrumentation. Moreover, as the field within the region of interest increases, the efficiency of a traditional permanent magnet decreases, with an increasing level of the stray field outside the gap.